Blender does not automatically create manifold when a space is enclosed by faces
Manifold geometry just means it’s watertight. Your printer should automatically translate from a sort of paper model to something solid. It does this by working with the face normals. If the add-on thinks the geometry is not manifold, there might be a hole in it somewhere, or maybe some of the face normals are oriented backwards. Try selecting everything, then using “Merge by distance” to merge verses that are in the same mathematical position but are not connected. After that, with success or failure, try re-calculating all the normals to the outside.